1. Field of the Invention
The present invention relates to an oxygen sensor which detects an air fuel ratio or the like of an automotive engine.
2. Description of Related Art
When the air fuel ratio of an internal combustion engine for a vehicle is not proper, not only the energy (fuel) is lost but also an air pollution is caused. Therefore, the air fuel ratio is detected by using an oxygen sensor. The-oxygen sensor includes an electric chemical cell by installing electrodes on both sides of a solid electrolyte which is an oxygen ion conducting body. Exhaust gas is introduced in one side of the electrode and the air is introduced in the other side. Thus, the air fuel ratio of the internal combustion engine is detected from a potential difference between the electrodes produced by a concentration difference. In the solid electrolyte mentioned above, in order to produce oxygen ion conduction, high temperature is required and the above-mentioned cell is heated by using a heater.
As shown in FIG. 4, oxygen sensor 90 has oxygen detecting element 91 which has tubular shape with a closed end forming an electric chemical cell, and housing 92 holding oxygen detecting element 91. Housing 92 has body portion 93 having flange 931 at an approximately middle portion. Body portion 93 has element covers 941 and 942 being inserted into an exhaust air passage at a lower portion thereof and protection covers 951 through 953 contacting the air at the upper portion thereof.
Oxygen detecting element 91 is held at body portion 93 through insulating material 932. In the inside of oxygen detecting element 91, heater unit 96 is installed. Heater unit 96 includes, heat generating body 962, made of NICHROME wire, helically wound on the outer circumferential surface of insulator 961 porous insulating layers (not shown in figure) covering the outside of heat generating body 962 (see Japanese Patent Application Laid-open No. 58-172543). NICHROME is a registered trademark of the Driver-Harris Wire Company and relates to alloys of nickel, chromium and iron containing less than 30% iron.
In the method for winding the heat generating body on the outer circumferential surface, an assembly work is easy and heating efficiency is good. The reason why heat generating body 962 is covered with the insulating layers is to obtain an electric insulation with the inside electrode of oxygen detecting element 91.
On the inside portion of insulator 961, a lead hole inserting a lead wire not shown in figure is made and heat generating body 962 is connected to the lead wire and the lead wire is connected to an electric supply line through the lead hole. That is, a lead wire drawing-out hole communicating with the lead hole of the inside from the outer circumferential surface of insulator 961 is made. The lead wire is drawn out and an end portion of the lead wire and an end portion of the heat generating body are welded.
However, the following problem exists in the related art oxygen sensor. Since the oxygen sensor using for a vehicle is exposed under severe vibration for a long time by actuating automobiles, a joint portion of the heat generating body and the lead wire can be broken undesirably. Although a heat generating body and a lead wire such as a NICHROME wire or the like a NICHROME wire and a lead wire are connected firmly by being welded, they can be often broken due to vibration.